Re: [PATCH bpf-next] libbpf: Add bpf_object__rodata getter function

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Mar 26, 2020 at 8:18 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> This adds a new getter function to libbpf to get the rodata area of a bpf
> object. This is useful if a program wants to modify the rodata before
> loading the object. Any such modification needs to be done before loading,
> since libbpf freezes the backing map after populating it (to allow the
> kernel to do dead code elimination based on its contents).
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
>  tools/lib/bpf/libbpf.c   | 13 +++++++++++++
>  tools/lib/bpf/libbpf.h   |  1 +
>  tools/lib/bpf/libbpf.map |  1 +
>  3 files changed, 15 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 085e41f9b68e..d3e3bbe12f78 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1352,6 +1352,19 @@ bpf_object__init_internal_map(struct bpf_object *obj, enum libbpf_map_type type,
>         return 0;
>  }
>
> +void *bpf_object__rodata(const struct bpf_object *obj, size_t *size)

We probably don't want to expose this API. It just doesn't scale,
especially if/when we add support for custom sections names for global
variables. Also checking for map->mmaped is too restrictive. See how
BPF skeleton solves this problem and still allows .rodata
initialization even on kernels that don't support memory-mapping
global variables.

But basically, why can't you use BPF skeleton? Also, application can
already find that map by looking at name.

> +{
> +       struct bpf_map *map;
> +
> +       bpf_object__for_each_map(map, obj) {
> +               if (map->libbpf_type == LIBBPF_MAP_RODATA && map->mmaped) {
> +                       *size = map->def.value_size;
> +                       return map->mmaped;
> +               }
> +       }
> +       return NULL;
> +}
> +
>  static int bpf_object__init_global_data_maps(struct bpf_object *obj)
>  {
>         int err;
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index d38d7a629417..d2a9beed7b8a 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -166,6 +166,7 @@ typedef void (*bpf_object_clear_priv_t)(struct bpf_object *, void *);
>  LIBBPF_API int bpf_object__set_priv(struct bpf_object *obj, void *priv,
>                                     bpf_object_clear_priv_t clear_priv);
>  LIBBPF_API void *bpf_object__priv(const struct bpf_object *prog);
> +LIBBPF_API void *bpf_object__rodata(const struct bpf_object *obj, size_t *size);
>
>  LIBBPF_API int
>  libbpf_prog_type_by_name(const char *name, enum bpf_prog_type *prog_type,
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 5129283c0284..a248f4ff3a40 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -243,5 +243,6 @@ LIBBPF_0.0.8 {
>                 bpf_link__pin;
>                 bpf_link__pin_path;
>                 bpf_link__unpin;
> +               bpf_object__rodata;
>                 bpf_program__set_attach_target;
>  } LIBBPF_0.0.7;
> --
> 2.26.0
>